The present invention relates to a two-phase fluid flow distribution system and method for a parallel flow evaporator or condenser and, more particularly to a system and method that achieve uniform distribution of the two-phase flow within parallel microchannel heat transfer passages and increase system performance by integrating an orientation-insensitive, two-phase flow distribution device within the inlet manifolds of the microchannel heat exchanger passages.
Traditional evaporators, for instance those in the. refrigeration or air-conditioning industry, utilize a single series flow path or a small number of series flow paths in parallel where an external distributor is used to assure uniform flow within these series flow paths that are flowing in parallel. However, as the flow passages become increasingly smaller, the increased pressure drop associated with a series flow configuration typically requires that all the passages in the evaporator flow in parallel, rather than having some of the flow in series. This is especially true with microchannel evaporators where the passages are typically under 3 mm in hydraulic diameter. Evaporating or condensing refrigerant-to-air heat exchangers (also referred to as coils), as opposed to cold plates, generally consist of a plurality of thin tubes sandwiched by thin folded fins and are connected to and fed fluid by an inlet manifold, with the fluid being discharged to an intermediate manifold or outlet manifold. These manifolds are also commonly referred to as headers. FIGS. 1A and 1B show typical microchannel evaporators of the type discussed below.
The use of an external distributor to feed equal mass flow rate to each passage would he impractical and far too costly for a typical parallel flow microchannel evaporator due to the large number of parallel paths. Therefore, some method is needed to assure that the mass flow rate of the fluid being evaporated is uniformly distributed among all the parallel flow passages that are directly connected to the inlet manifold as shown in FIGS. 1A or 1B. Furthermore, a two-phase flow distribution device that is also not orientation-specific, that is one that does not require gravity to separate the liquid and vapor for proper operation, is needed. The current lack of an effective, manufacturable and reasonably priced, approach has prevented the widespread use of anything but single-bank (also referred to as single-pass) up-flow microchannel evaporators (as shown in FIG. 1A). Furthermore, even when orientation, and therefore the effect of gravity is used to aid in the flow distribution, the resulting distribution is less than ideal.
In a completely parallel-flow condenser, such as a microchannel condenser, the flow distribution is far simpler than for the case of evaporation of a two-phase mixture. This is due to the superheated vapor at the condenser inlet consisting entirely of vapor, and therefore the entire flow in the condenser has consistent physical properties, such as density and viscosity, This superheated floss enters the parallel-flow condenser passages for cooling and subsequent condensation in the, passages (after distribution, into these passages). In a parallel-flow evaporator, however, a two-phase mixture of liquid and vapor, due to the flashing of the refrigerant at the upstream throttling valve, must be equally distributed to each of the passages for optimum performance. As a result, the two-phase mixture distributed to each of the parallel flow passages in an evaporator tend to separate due to differences in the physical properties of the liquid and vapor (liquid and vapor have different physical properties, such as density, wettability and viscosity). The differing properties of the flowing liquid and vapor result in differences in, among other things, the effect of inertial and gravitational forces on the vapor and the denser liquid, resulting in flow maldistribution in a conventional parallel flow evaporator configuration as shown for example in FIG. 1A.
Therefore, while microchannel heat exchangers have largely replaced legacy tube-fin heat exchangers used, for automotive condensers and residential heating, ventilation, air-conditioning, and refrigeration (HVAC-R) condensers due to their increased heat transfer performance, improved form factor, lightweight design and reduced cost, current microchannel evaporators suffer from maldistribution within the manifolds due to the nature of the two-phase fluid flow in the inlet, manifold. This maldistribution causes a decrease in heat transfer performance, thus mitigating the advantages of a microchannel evaporator. For this reason, microchannel evaporators are not typically used in the HVAC-R industry, and tube-fin coils are still the predominant technology for HVAC-R evaporators.
Currently, most HVAC-R systems that use microchannel heat exchangers as an evaporator only do so when a dual-mode air conditioning/heat pump system is being operated as a heat pump. In heating mode, maldistribution of the vapor in the evaporator (outdoor coil) can be tolerated because heating mode performance is generally less critical than cooling mode performance. When this same dual-mode system is operated in the more challenging cooling mode, however, that same outdoor coil is the system condenser and provides improved performance when compared to a conventional tube-fin condenser coil. Therefore, most systems that incorporate a microchannel heat exchanger in HVAC-R, applications utilize the microchannel heat exchanger as the condenser for a single-mode air conditioner, or as the outdoor coil in a dual-mode air conditioner/heat pump. For dual mode HVAC-R systems the outdoor coil operates as the condenser in air conditioner mode and operates as the evaporator when in heating mode.
Another issue with microchannel heat exchangers (and parallel passage heat exchangers in general) used in vapor compression and other two-phase heat transfer systems is that the evaporator or condenser may consist of multiple parallel-path heat exchangers (referred to as “banks” of the overall heat exchanger) where the exhaust header of the first heat exchanger (first bank) is connected. to the inlet header of the next heat exchanger (second bank,) and so on as shown in FIG. 1B for a three bank or three pass heat exchanger. Flow maldistribution is an ongoing problem with banks of heat exchangers operating as evaporators but can also be a problem on the second and subsequent banks of a condenser due to the condensation of some of the working fluid in the first bank becoming maldistributed in the inlet manifold of the second and subsequent banks where a two-phase mixture exiting the first bank of condenser must flow into the inlet manifold (or header) of the next bank of the condenser. in a condenser per se, there is no need to integrate an inlet distributor since the inlet flow is entirely vapor; subsequent banks (subsequent inlet headers) will, however, have a combination of liquid and vapor refrigerant requiring proper flow distribution. The lack of an effective two-phase flow distributor decreases performance of the heat exchanger due to the maldistribution affects incurred.
In the past, two-phase distribution devices, such as either a tube with holes (FIG. 3A) or porous medium (FIG. 3B), have been integrated within the inlet manifold of the microchannel heat exchanger in an effort to increase the uniformity of two-phase flow to the plurality of tubes as shown in FIG. 2A. This type of configuration relies on gravitational forces to separate the liquid and vapor and in the case of a porous medium to keep the liquid from making direct contact with the porous medium and unfavorably saturating the porous medium with liquid. This methodology has not, however, provided uniform two-phase flow in down-flow evaporator configurations or in the down-flow portions of multi-bank evaporators or condensers such as shown in FIG. 1B. The absence of an effective distributor mechanism both at the inlet and also between sequential banks within the heat exchanger results in a maldistributed flow and degrades both the heat exchanger and system performance. In the past, this has led to complicated and expensive attempts to avoid multiple bank heat exchangers or to configure up-flow-only evaporators or some other manifold configuration where gravity is used to keep the liquid away from the porous medium. If liquid contacts the porous medium, then capillary action draws the liquid into the pores, starving the downstream portions of the manifold from achieving proper liquid distribution by preventing the liquid from traversing the full length of the inlet manifold. For instance, manufacturing methods have been developed that bend the plurality of tubes to retain a single-bank heat exchanger rather than directly address the issue of flow maldistribution. That approach does not provide a solution for the underlying problem and fails to allow for the creation of compact multiple bank heat exchanger configurations such as shown in FIG. 2B, instead forcing the use of larger, more expensive to manufacture, and more cumbersome multi-bank evaporators of the type shown in FIGS. 4A and 4B, where all evaporation occurs in up-flow and with gravity assisting in the performance of the flow distribution device.
The need for up-flow only evaporator configurations for proper operation of the flow distributing device, that is the need for using gravity to separate the liquid and vapor and prevent liquid from saturating the porous medium, means that the compact multi-bank heat exchangers where up-flow and down-flow patterns are used in alternating tube banks, would have flow distribution issue in the down-flow banks. For example, the simplest, most compact and cost-effective way to create a multi-bank microchannel evaporator is to pass refrigerant from one bank to the next as shown in FIG. 1B. This method simply allows the flow to progress further down. the intermediate portion of manifold 102′ as shown by the flow arrow 121′, where the refrigerant upward flow in Bank 1 is denoted by flow arrow 120′ (and upward flow stops at. barrier 108′) and then the flow progresses down manifold 102′ flowing downward in Bank 2, as shown by flow direction arrow 122′ (down-flow stops at barrier 118′). Flow then progresses through the intermediate portion of the lower manifold 103′ as shown by the flow arrow 123′, where the refrigerant upward flow in Bank 3 is denoted by flow arrow 124′ (and up-flow stops at end cap 115′) and then the flow exits at 106′. Up until now, there have been no effective flow distribution methods for the downward flow section(s) of such a heat exchanger and therefore other costlier approaches have had to been employed. For example, to assure all parallel passages are in an up-flow orientation, a jumper tube is used (as shown, for example, in FIGS. 4A and B as 488 and 489) so that all flows are up-flows and therefore gravity can be used to enable the two-phase flow distribution device to properly feed the parallel passages, since good flow distribution is always necessary in the inlet manifold to the parallel-flow microchannel passages.
The present invention addresses these problems with a novel system and method of improving two-phase distribution in microchannel evaporators with single or multiple banks without creating significant pressure drop and without the need to only operate in a specific orientation. We have improved the two-phase flow of the evaporator by incorporating a porous medium along with an impervious passage (or surface coating on the porous medium) as part of the evaporator manifolds for both single and multi-bank arrangements. We have found that our invention uniformly distributes the liquid phase throughout the header and mitigates gravitational and inertial separation effects in the inlet or intermediate manifolds. Within the microchannel evaporator, the device can be integrated into the manifolds, between passes, between banks or any combination thereof.
We have performed experiments to verify that the use of a combination of a porous medium which incorporates a non-permeable surface with discreet openings to allow the liquid to evenly migrate into the porous surface provides an improved flow distributor in both up-flow and down-flow configurations as will be described in greater detail below. In these thermal images, the dark areas represent the presence of liquid or two-phase flow, and thus areas where good flow distribution has been achieved is shown when these areas are present across the length of the evaporator.
In our invention, a conventional thermostatic expansion valve (TXV) or other type of metering device is provided upstream of the evaporator to effectively reduce the pressure of the fluid so as to create the two-phase conditions. While, in general, this configuration can be used with any parallel passage heat exchanger configuration employing an inlet header, our invention will also accommodate microchannel evaporators or condensers formed by using flat tube parallel passages separated by folded fins.
In our invention, a single manifold can contain multiple distribution arrays to promote uniform distribution between banks (also referred to “passes”) of the evaporator. These novel distribution arrays can be separated by a flow obstruction and allow for alternating upward and downward flow of the fluid. The banks of the microchannel evaporator can contain an identical or varying number of tubes.
Additionally, multiple bank evaporators can be connected by using a single manifold incorporating our unique unibody approach that directly takes fluid from one bank to the next without the use of a jumper tube. These manifolds use the distribution method of our invention to reduce maldistribution and can be manufactured from multiple parts brazed together or as a single tube extrusion.